Spectroscopic properties of bismuth doped borate, silicate and phosphate glasses have been reinvestigated in this work. It shows the typical decay time of Bi3+ is around 500ns rather than 2.7-to-3.9 μs reported by Parke and Webb at room temperature. Introduction of higher content either alkali or alkali earth into borate glasses favors the Bi3+ emission. As the contents increase excitation peak shifts regularly red while emission peak shows reverse trend. This, as revealed by Huang-Rhys factor, is due to the weakening of coupling between bismuth and glass host, and it can be interpreted within the frame of configurational coordinate diagrams. Differently, as bismuth concentration increases, both the excitation and emission shift red. The unknown origin of red emission from bismuth doped calcium or magnesium phosphate glass has been identified as Bi2+ species on the basis of excitation spectrum and emission lifetime particularly after comparing with Bi2+ doped materials. No near infrared (NIR) emission can be detected in these glasses within instrument limit.
©2012 Optical Society of America
Pandora’s box of bismuth has been opened slowly since first finding of strange red luminescence from BaSO4:Bi in the end of nineteenth century, about a century later confirmed due to a less known state Bi2+ . The opening has brought us surprises, hopes and sometimes confusions. This mainly comes from the diversity of the element in valence states and easy formation into clustering entities [2–6]. It, on the other hand, endows the element of bismuth with unpredicted optical properties. For instance, the peculiar divalent bismuth when stabilized in crystals can have ultraviolet or blue absorptions and intense orange or even red luminescence [1,3,7–11]. This enables a potency to improve color rendering index of commercial white light LEDs when combined. Oscillation strength of 2P1/2→2P3/2 of Bi2+ absorptions was reported even larger than 0.03 , comparable to Eu2+, though the transition is theoretically forbidden in parity. It’s possibly because the transition probability has been strongly promoted by intense coupling to surrounding crystal field as the ion of Bi2+ is built into the condensed solids. For another instance, very recently, polycation Bi53+ was found unexpectedly luminescing in mid infrared . It is also weird the luminescence quenches in chloride glass but not crystalline sample at room temperature [6,12]. For one more instance, in last decade a new family of bismuth doped materials has been found and soon become a hot research area because of attractive optical properties [5,13–17]. It features in extraordinary absorptions spectrally ranging from UV, visible to near infrared (NIR), and superbroad emissions in 1000 to 1600nm [13–21]. And it can provide net optical gain in the regime where traditional rare earth cannot work, therefore it has been considered for developing new type of laser devices and next generation superbroad fiber amplifiers [22–30]. So far, inspiring reports have been published subsequently on the devices [26–30]. Nevertheless, there is still no direct and convincing evidence to prove which exact valence state of bismuth is responsible for the NIR emission, though a consensus has been made that the valence should be lower than + 2 [5,9,31–34]. Only recently, bismuth doped glasses have been demonstrated to convert lights in 200-900nm uniquely into NIR lights, implying a possible application as solar spectral converters or concentrator . The NIR bismuth emission centers can be even in situ created by femtosecond laser illumination besides treating in reducing conditions, such as CO [9,36,37]. In some cases different bismuth species coexist in the sample and they could be easily converted into each other. Coexistence of non- or multiple- NIR emission centers can result in additional optical loss of bismuth based fiber devices, and deteriorate the beam quality of output laser [23–25]. In bismuth doped pure silica or aluminosilicate fibers, Bufetov et al have clearly identified that there exist different but single types of bismuth active centers (BAC) [26–29]. BAC in doped pure silica fiber, when uncooled, demonstrates high laser efficiency of 58%; another type of BAC in aluminosilicate fiber exhibits lower efficiency of ~20% [27,28]. Though in the presence of single type of NIR emission centers the efficiency of the lasers can be different depending on their properties, it is desirable to precipitate a single type of NIR emission centers in the materials to improve the quality of laser beam. For that, more knowledge on how to selectively stabilize a specific Bi emission species for activity at a specific spectral range in different glasses will be preferred. In other content, this can help understand the puzzling nature of NIR emission in a different way.
Normally, valence states of species could be identified easily with traditional instrumentations such as XPS, EXAFS or EELS. However, these become less sensitive or reliable for heavy metal especially for bismuth. Possibly due to the easy conversion between bismuth species and lower melting point of bismuth, sometimes artifact can be caused unintentionally during measurements. For instance, we found after XPS the measurement bismuth doped germanate glass has changed into black from original transparent brown. When measuring TEM and EELS of bismuth particles, the particles are readily melted as exposed directly to electron beams. In view of these, alternative techniques should be consulted, of which optical spectroscopy is mainly concerned in this work. This is motivated by the facts that no visually artificial information could be produced during our optical measurements and the technique is hypersensitive to the valence state of activators and the changes of local environment around them. So it will be meaningful to accumulate spectroscopic data of different kinds of bismuth species. As one of systematic studies, here in this work we will focus mainly on Bi3+ doped glasses.
Since last century Bi3+ doped crystals have been noticed as one of series works on fluorescence properties of ions with ns2 configuration [2,4,38–43]. Unlike the Bi3+ doped crystals, Bi3+ doped glasses have been paid less attention partially due to low luminescence efficiency. To today, Bi3+ doped borate, silicate, phosphate and germanate glasses have been reported [39–43]. However, these reports mainly emerged between 1970s and 1990s and concentrated on absorption and emission spectra [39–43]. Except these, no systematic studies have been performed. Electronic configuration of Bi3+ is [Xe]4f145d106s2. The ground state is 1S0 with 6s2 configuration, and the excited states from 6s6p configuration are 3P0, 3P1, 3P2 and 1P1 in a sequence of energy increasing. The transitions 1S0→3P0 or 3P2 are spin forbidden and 1S0→3P1 or 1P1 are lifted by spin-orbit coupling. So, the latter two have relatively higher absorption strength than the former two. Backward radiative transition 3P1→1S0 is Laporte allowed and the decay time usually is counted between 10−6 and 10−8s. For an instance, Blasse et al found LaBO3: Bi3+ and CaSO4:Bi3+with lifetimes of 0.8μs and 60( ± 10) ns, respectively [2,4]. Reisfeld and Boehm reported Bi3+ doped germanate glass with a decay time of 350ns at room temperature . However, Parke and Webb found the lifetime of Bi3+ emission varying between 2.7 and 3.9μs in borate, silicate and phosphate glasses . The temporal character of the glasses if it was true would be comparable to Bi2+ doped crystals, e.g. Sr2P2O7:Bi2+ (less than 10μs) [7–11]. This increases the uncertainty to accurately assign a transition of bismuth in specific valence. To avoid this, plus in view of the striking deviation from the reports on Bi3+ doped materials and the lack of complete spectroscopic data, it seems necessary to initialize the reinvestigation partly to redefine the lifetime of Bi3+ in these glasses.
Here in this work, we have found the emission lifetime of Bi3+ doped borate, silicate, phosphate glasses actually does not show obvious divergence from doped phosphors and germanate glass, and it is much shorter than Bi2+ lifetime, and in the meantime, we have unveiled the origin of the weak red emission from bismuth doped calcium phosphate glass, which could not be identified by Parke and Webb at that moment . Moreover, the red emission can be observed in bismuth doped magnesium phosphate glass. In the case of accommodation with either alkali or alkali earth oxide none of NIR emission centers can survive in borate and silicate glasses when prepared in air.
2. Experimental procedure
All glass samples were melted in air on a scale of 30g by a conventional melting and quenching method. As a reference, blank specimen was prepared in parallel for each series of glasses. Analytic reagents H3BO3, Li2CO3, Na2CO3, K2CO3, MgO, CaCO3, SrCO3, BaCO3, SiO2, P2O5, Al(OH)3 and Bi2O3 were selected as raw materials. Samples were weighed according to molar compositions xMzO·(100-x-y)B2O3·yBi2O3 (M = Li, Na, K, Ba; x = 5, 10, 15, 20, 25, 30, 33, 35; y = 0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5), 30Na2O·(70-x)SiO2·xBi2O3 (x = 0, 0.5) and 50MzO·(50-x)P2O5·xBi2O3 (M = Na, Mg, Ca, Sr, Ba; x = 0, 0.5). Each borate sample was melted in platinum crucibles at 1200°C for 20 min. The samples of phosphate and silicate were melted at 1100°C for 30 min and at 1400 °C for 6h, respectively. Samples 30Li2O·69.5B2O3·0.5Bi2O3 and 35Li2O·64.5B2O3·0.5Bi2O3 are easily devitrified and 35K2O·64.5B2O3·0.5Bi2O3 is quite hygroscopic in ambient. So, these are neglected in this study. Samples were annealed before cut and polished for optical measurements.
Static excitation and emission spectra and emission decay spectra were measured with a high resolution spectrofluorometer Edinburgh Instruments FLS 920 equipped with a red sensitive single photon counting photomultiplier (Hamamatsu R928P) in Peltier air-cooled housing. A microsecond pulsed xenon flashlamp μF900H with an average power of 60W, which can measure decays from 1μs to 10s, was used to measure the decay curves of red emission from bismuth doped phosphate glasses. For measuring lifetime of Bi3+ emission in borate and silicate glasses, a hydrogen filled nanosecond flashlamp nF900 applicable to a lifetime of 100ps to 10μs was employed. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vector 33 spectrometer on samples embedded in KBr pellets. All the measurements were performed at room temperature.
3. Results and discussion
3.1 Bi3+ doped borate and silicate glasses
Parke and Webb examined  the absorption, emission and decay times of bismuth doped x Na2O·(100-x)B2O3 (x = 10, 15, 20, 30, 35), 20Li2O·80B2O3, 20K2O·80B2O3, 3Na2O·7SiO2 and MxO·P2O5 (M = Li, Na, Mg, Ca, Sr, Zn) glasses. For bismuth doped borate glasses 10Na2O·90B2O3 and 15Na2O·85B2O3, they found brown coloration though not uniformly distributed throughout the samples. We extended the study to a wider range xMzO·(100-x-y)B2O3·yBi2O3 (M = Li, Na, K, Ba; x = 5, 10, 15, 20, 25, 30, 33, 35; y = 0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5), however, we could only get clear colorless samples in the series. For bismuth doped Na2O-B2O3 binary glasses, only weak luminescence exists when soda content x increases from 5 to 20, and it is intensified markedly as x keeps increasing to 35, as can be seen in Fig. 1 . This basically agrees with what Parke and Webb found in the glasses. They noticed that, only when x is larger than 15 can relatively stronger fluorescence be recorded. They also found absorption peak depends more on the concentration than the nature of alkali ion. For instance, the absorption peaks at 230nm (43500cm−1 due to the transition of 1S0→1P1) same in 20Li2O·80B2O3, 20Na2O·80B2O3 and 20K2O·80B2O3 glasses. However, it moves towards lower energy side (225nm (44400 cm−1) → 240nm (41700cm−1)) as the concentration of alkali increases from 15 to 35 in sodium borate glasses . Nevertheless, this comment does not hold all true for excitation and emission peaks, as we will discuss below (see Figs. 1-3 ). Because of instrumentation limit, excitation in the spectral range of 200 to 250nm cannot be recorded meaningfully.
Figure 1 denotes the excitation and emission spectra of xNa2O·(99.5-x)B2O3·0.5Bi2O3 (x = 25, 30, 35) glasses. When x increases, emission and excitation peak exhibits different behaviors. The emission shifts blue though slightly (430nm → 428nm → 426nm), while the excitation obviously shifts red (309nm → 315nm → 319nm, see Fig. 1). At the same time stokes shift decreases (9106cm−1 → 8382 cm−1 → 7874 cm−1). Same scenario does appear in bismuth doped Li2O-B2O3, K2O-B2O3 and BaO-B2O3 glasses. Results are collected in Table 1 . From Figs. 1-2 and Table 1 we can notice that changes of alkali or alkali earth content doesn’t cause significant shift of emission peaks for the glass series of BaO-B2O3 and Na2O-B2O3 particularly when comparing with the cases of Li2O-B2O3 and K2O-B2O3. For glasses with equal content of alkali oxide, vis. 25M2O·74.5B2O3·0.5Bi2O3 where M stands for Li, Na and K, the emission of Bi3+ shifts towards low energy in sequence of Li → Na → K. Similar case happens in NIR emissive bismuth doped glasses [5,19–21,44].
Figure 3(a) denotes excitation and emission spectrum of 30Na2O·69.5SiO2·0.5Bi2O3 glass. It reveals one excitation peak at 299nm and one emission peak at 395nm for the transparent colorless sample (see inserted image). The emission peak differs from 385nm by Parke and Webb . The Stokes shift is 8129cm−1, comparable to 8382cm−1 of 30Na2O·69.5B2O3·0.5Bi2O3 but smaller than 9478cm−1 of 30K2O·69.5B2O3·0.5Bi2O3 glass.
When measuring lifetime of Bi3+ in borate and silicate glasses, Parke and Webb selected pulsed exciting light with pulse width, that is, FWHM of 200ns and they obtained decay time ranging from 2.7 to 3.9μs . In this study based on the technology of time correlated single photon counting, we chose hydrogen filled gas nanosecond flashlamp with typical pulse width of 1ns. Typical decay curve of Bi3+ emission of doped silicate glass is illustrated as Fig. 3 (b). And it complies well with single exponential decay equation, fit to which, respectively, yields 427ns and 475ns for the two glasses. The rest data are summarized as column “τ1” in Table 1. The typical lifetime of Bi3+ doped borate and silicate glasses is around 500ns.
Besides types and concentrations of alkali or alkali earth, dopant concentration also affects excitation and emission of Bi3+ as shown in Fig. 4 . Increase in concentration leads to weakening intensities of emission and excitation peaks and simultaneously the redshift of these peaks. The excitation peak varies from ~309nm to ~330nm, and the emission moves from ~425nm to 440nm, perhaps due to self absorption of Bi3+. Lifetimes lie between 470ns and 516ns. This is very similar to NIR emission Bi doped germanate glasses, but the dependence of optical properties on bismuth content is much stronger. For example, increase of bismuth content from 0.01% to 2% even leads to the shift of emission from 1100nm to 1310nm .
3.2 Mechanism for the compositional dependence of Bi3+ emission in the borate and silicate glasses
On the basis of spectroscopic data reported on Bi3+ doped crystals and glasses [2,4, 38–43], it can be noticed that, the first excitation band clearly corresponds to the dominate absorption transition from 1S0 to 3P1 at either room temperature or even lower temperature for instance 4.2K , and the nature of emission transition strongly depends on temperature. After electrons have been raised to some vibrational level of the excited state of 3P1, part of them will, at lower temperatures, relax to the lower lying 3P0 state via a radiationless transition and therefore the forbidden 3P0→1S0 transition can be observed. However, at higher temperatures, electrons on the state of 3P0, if there has ever been any electron populated on the state, will tend thermally depleted to 3P1. As a consequence the emission due to the transition of 3P1→1S0 preponderates [39–41]. So, at room temperature the excitation peak in borate and silicate glasses (see column “λex” of Table 1) is attributed to the transition of 1S0→3P1, and the emission is to 3P1→1S0 (see column “λem” of Table 1).
Inspecting Figs. 1-2 and Table 1 shows that, as stated earlier for Bi3+ doped alkali or alkali earth borate binary glasses, excitation peak regularly shifts red and emission peak shifts blue with concentration of alkali or alkali earth increasing. As a result, Stokes shift decreases. How does all this happen? To elucidate it, information on glass microstructural changes is helpful.
Figure 5 shows FTIR spectra of xBaO·(99.5-x)B2O3·0.5Bi2O3 (x = 20, 25, 33) and xNa2O·(99.5-x)B2O3·0.5Bi2O3 (x = 25, 30, 35) glasses. The spectra reveal the almost same trend, that is, as the content of modifier ions increases, amount of BO4 anion groups grows at the expense of BO3, possibly due to the introduction of additional oxygen along with the modifiers. This should be the reason why the Stokes shift decreases. It’s known that the transitions of Bi3+ occur between 6s2 and 6s6p configurations [2, 3, 8–43]. Unlike rare earth, they are easily influenced by ligand fields around Bi3+ ion. Coupling strength of electron and phonon can be qualitatively measured by Huang-Rhys factor S and it can be estimated from S = 1/2(SS/ħω + 1), where ħω is the maximum phonon energy, and SS means Stokes shift [8, 11]. Here, if the maximum phonon is taken as ~1350cm−1 corresponding to typical B-O stretching vibration of trigonal BO3 unit in borate glasses, S can then be calculated and listed in Table 1. Based on the data above, schematic configurational coordinate diagrams are constructed for two extreme cases: (a) lower content of alkali oxide with higher S and (b) higher content of alkali oxide with lower S, as denoted by Fig. 6 (a) and 6(b), respectively. For each series of doped samples M2O-B2O3 (M = Li, Na, K), for case (a), Bi3+ ion will experience stronger interaction with environment, and it leads to a bigger offset ΔQ between 1S0 and 3P1, and shorter wavelength absorption, longer wavelength emission, and hence larger Stokes shift. Case (b) is opposite to case (a) and it results in smaller Stokes shift. Figure 6 can also qualitatively apply to Bi3+ doped glass of BaO-B2O3.
3.3 Red emission from bismuth doped phosphate glasses
Parke and Webb noticed red emission from bismuth doped calcium phosphate and did not perform more optical measurements . But they did suggest the red emission seems unlikely to be the simple transition of 3P1→1S0 of Bi3+. The origin of the emission cannot be identified at that time. Here, beside calcium phosphate glass, the red emission can be observed in magnesium phosphate glass, though relatively weaker. It cannot be recorded in 50Na2O · 49.5P2O5 · 0.5Bi2O3, 50Sr2O · 49.5P2O5 · 0.5Bi2O3, and 50BaO · 49.5P2O5 · 0.5Bi2O3 glasses when prepared in air.
Figure 7 depicts the excitation, emission and decay curves of 49.5CaO · 50P2O5 · 0.5Bi2O3 (CAB, hereafter) glass. Red emissions indeed were observed in the glasses and they slightly depend on excitation wavelengths, as curves c-d of Fig. 7(a) show. Excitations into 285nm, 345nm and 535nm, respectively, lead to an emission peaking at 712nm, 709nm and 709nm. Accordingly, excitation spectra are different though all composed of three peaks when the monitored emissions change (see curves a and b). For the emission at 712nm, excitation peaks lie at 285, 340 and 520nm; and for the emission at 640nm, the peaks at 285nm, 331 and 495nm. And the peak at 285nm always is the strongest. Decay curves were measured for different situations (see exemplarily curves a-c of Fig. 7(b)). For the emission at 712nm and the excitation at 285nm, decay curve follows well simple exponential equation and it produces a lifetime of 4.417μs. Similarly, lifetimes are 6.041μs and 7.082μs for the cases of Em709nm@Ex345nm and Em704nm@Ex500nm. Figure 7 implies the existence of more than a single type of bismuth emission centers in the glass.
The excitation, emission and decay curves of 49.5MgO · 50P2O5 · 0.5Bi2O3 (MPB) glass are quite analogous to the glass CPB. The emission peaks of the MPB glass appear at shorter wavelength when compared to CPB glass, and they strongly depend on excitation wavelengths. Upon excitations at 281, 335 and 507nm the emission shifts from 694nm, 681nm to 676nm. Excitation spectrum of the emission at 694nm comprises three peaks at 281nm, 335nm and 507nm, similar to the glass CPB. Lifetimes are measured to be 4.799μs, 6.351μs and 7.182μs for the cases of Em694nm@Ex281nm, Em681nm@Ex335nm and Em676nm@Ex507nm, respectively.
3.4 The origin of red emission from bismuth doped phosphate glasses
The features of excitation and emission spectra and fluorescence lifetime of CPB and MPB glasses are distinctly different from Bi3+ doped either glasses or crystals. The excitation spectrum spans from ultraviolet to visible, and the emission appears at longer wavelength of ~700nm, and the lifetime is about 10 times longer than Bi3+, and about 100 times shorter than bismuth NIR emission centers (typically several hundred microseconds) [2,4,13–21,31–37]. This means that red luminescence center in bismuth doped CPB and MPB glasses intrinsically differs from Bi3+ or bismuth NIR emission centers. It, however, highly resembles Bi2+. Table 2 summarizes the spectroscopic data of Bi2+ doped crystals and glasses reported so far in literatures [1,3,7–11].
As shown in the table, three absorption peaks corresponding to 2P1/2 → 2P3/2 (1), 2P1/2 → 2P3/2 (2) and 2P1/2 → 2S1/2, can be usually detected in Bi2+ doped compounds. Accordingly, emission occurs via 2P3/2 (1) →2P1/2 [1,3,7–11]. The absorption of 2P1/2 → 2S1/2 lies between 231 and 286nm, and the two absorptions of 2P1/2 → 2P3/2 (1) and 2P1/2 → 2P3/2 (2) can be separated from each other in the crystals and they appear in the spectral range of 380 to 648nm. The emission of 2P3/2 (1) →2P1/2 appear in 586 to 716nm with a lifetime varying between 8 and 25μs. In the compound of Ca1.994P2O7:0.3% Bi, since there is more than one type of Bi2+, two absorption peaks of the 2P1/2 → 2S1/2 transition were observed at 237 and 281nm . This similarity encourages us to assign the absorptions at 285nm and 340nm in CPB glass and 281nm and 335nm in MPB glass to the transition from 2P1/2 to 2S1/2, since as mentioned above more than one type of Bi2+ exists in these glasses. The lifetime of CPB and MPB glasses is in the same order as Bi2+ in crystals. We attribute the broad absorption covering the range of 400 - 650nm to 2P1/2 → 2P3/2 (1) and 2P1/2 → 2P3/2 (2) and the red emission of CPB and MPB glasses to 2P3/2 (1) →2P1/2. Unlike Bi2+ doped crystals, the transitions 2P1/2 → 2P3/2 (1) and 2P1/2 → 2P3/2 (2) cannot be well resolved, and it is possibly because Bi2+ in the glasses experiences stronger coupling strength from lattice especially as comparing CPB glass to the crystal sample with almost same composition Ca1.994P2O7:0.3% Bi . This may also explain why the emission from CPB glass locates at a longer wavelength than the crystal.
When checking over Bi doped alkali or alkali earth borate glasses and phosphate glasses no NIR luminescence can be detected in the case of 808nm excitation at room temperature. This agrees with foregoing investigations and it confirms again that higher content of alkali or alkali earth oxide is harmful to the stabilization of NIR emission bismuth centers though it is favorable for the luminescence of Bi3+ or in some cases Bi2+.
In all, the reinvestigation on bismuth doped alkali or alkali earth borate, silicate and phosphate glasses has defined that the decay time of Bi3+ is typical ~500ns in borate and silicate instead of several microsecond reported by Parke and Webb, and it has revealed that Bi2+ ion is responsible for the red emission of bismuth doped calcium phosphate glass, the nature of which Parke and Webb confused about. The red emission corresponds to 2P3/2 (1) →2P1/2 of Bi2+ and it also occurs in magnesium phosphate glass. None of bismuth doped alkali or alkali earth borate, silicate and phosphate glasses can accommodate bismuth NIR emission centers, perhaps because of high content of alkali or alkali earth oxide in the glasses. It can, however, stabilize Bi3+ ion in borate and silicate glasses. The composition dependence of Bi3+ optical properties can be explained with the configurational coordinate models. Stronger coupling between Bi3+ and lattice can lead to redshift of emission and blueshift of excitation and eventually larger Stokes shift. In either magnesium or calcium phosphate glass only Bi2+ ion survives. Beside accumulation of spectroscopic data on different valent bismuth ions, this work can guide us on how to manipulate precipitation of some specific bismuth ions by modulating glass components.
This work was financially supported by the National Natural Science Foundation of China (Grant No. 51072060, 51132004, 51102096), Fundamental Research Funds for the Central Universities (Grant No. 2011ZZ0001), Guangdong Natural Science Foundation (Grant no. S2011030001349), Fok Ying Tong Education Foundation (Grant No. 132004) and Chinese Program for New Century Excellent Talents in University (Grant No. NCET-11-0158).
References and links
1. M. Hamstra, H. Folkerts, and G. Blasse, “Materials chemistry communications. Red bismuth emission in alkaline-earth-metal sulfates,” J. Mater. Chem. 4(8), 1349–1350 (1994). [CrossRef]
2. G. Blasse and A. Bril, “Investigations on Bi3 +-activated phosphors,” J. Chem. Phys. 48(1), 217–222 (1968). [CrossRef]
3. G. Blasse, A. Meijerink, M. Nomes, and J. Zuidema, “Unusual bismuth luminescence in strontium tetraborate (SrB4O7:Bi),” J. Phys. Chem. Solids 55(2), 171–174 (1994). [CrossRef]
4. D. van der Voort and G. Blasse, “Luminescence of CaSO4:Bi3+, a small-offset case,” J. Solid State Chem. 99(2), 404–408 (1992). [CrossRef]
5. M. Peng, G. Dong, L. Wondraczek, L. Zhang, N. Zhang, and J. Qiu, “Discussion on the origin of NIR emission from Bi-doped materials,” J. Non-Cryst. Solids 357(11-13), 2241–2245 (2011). [CrossRef]
8. M. Peng, N. Da, S. Krolikowski, A. Stiegelschmitt, and L. Wondraczek, “Luminescence from Bi2+-activated alkali earth borophosphates for white LEDs,” Opt. Express 17(23), 21169–21178 (2009). [CrossRef] [PubMed]
9. M. Peng, B. Sprenger, M. A. Schmidt, H. G. Schwefel, and L. Wondraczek, “Broadband NIR photoluminescence from Bi-doped Ba2P2O7 crystals: insights into the nature of NIR-emitting bismuth centers,” Opt. Express 18(12), 12852–12863 (2010). [CrossRef] [PubMed]
11. M. Peng and L. Wondraczek, “Orange-to-red emission from Bi2+ and alkaline earth codoped strontium borate phosphors for white light emitting diodes,” J. Am. Ceram. Soc. 93, 1437–1442 (2010).
12. A. N. Romanov, Z. T. Fattakhova, A. A. Veber, O. V. Usovich, E. V. Haula, V. N. Korchak, V. B. Tsvetkov, L. A. Trusov, P. E. Kazin, and V. B. Sulimov, “On the origin of near-IR luminescence in Bi-doped materials (II) Subvalent monocation Bi^+ and cluster Bi_5 ^3+ luminescence in AlCl_3/ZnCl_2/BiCl_3 chloride glass,” Opt. Express 20(7), 7212–7220 (2012). [CrossRef] [PubMed]
13. M. Peng, J. Qiu, D. Chen, X. Meng, I. Yang, X. Jiang, and C. Zhu, “Bismuth- and aluminum-codoped germanium oxide glasses for super-broadband optical amplification,” Opt. Lett. 29(17), 1998–2000 (2004). [CrossRef] [PubMed]
14. M. Peng, J. Qiu, D. Chen, X. Meng, and C. Zhu, “Superbroadband 1310 nm emission from bismuth and tantalum codoped germanium oxide glasses,” Opt. Lett. 30(18), 2433–2435 (2005). [CrossRef] [PubMed]
16. M. Peng, N. Zhang, L. Wondraczek, J. Qiu, Z. Yang, and Q. Zhang, “Ultrabroad NIR luminescence and energy transfer in Bi and Er/Bi co-doped germanate glasses,” Opt. Express 19(21), 20799–20807 (2011). [CrossRef] [PubMed]
18. X. Jiang and A. Jha, “An investigation on the dependence of photoluminescence in Bi2O3-doped GeO2 glasses on controlled atmospheres during melting,” Opt. Mater. 33(1), 14–18 (2010). [CrossRef]
19. G. Chi, D. Zhou, Z. Song, and J. Qiu, “Effect of optical basicity on broadband infrared fluorescence in bismuth-doped alkali metal germanate glasses,” Opt. Mater. 31(6), 945–948 (2009). [CrossRef]
20. Z. Song, Z. Yang, D. Zhou, Z. Yin, C. Li, R. Wang, J. Shang, K. Lou, Y. Xu, X. Yu, and J. Qiu, “The effect of P2O5 on the ultra broadband near-infrared luminescence from bismuth-doped SiO2-Al2O3-CaO glass,” J. Lumin. 131(12), 2593–2596 (2011). [CrossRef]
21. Z. Yang, Z. Liu, Z. Song, D. Zhou, Z. Yin, K. Zhu, and J. Qiu, “Influence of optical basicity on broadband near infrared emission in bismuth doped aluminosilicate glasses,” J. Alloy. Comp. 509(24), 6816–6818 (2011). [CrossRef]
22. E. Dianov, V. Dvoyrin, V. Mashinsky, A. Umnikov, M. Yashkov, and A. Gur'yanov, “CW bismuth fibre laser,” Quantum Electron. 35(12), 1083–1084 (2005). [CrossRef]
23. I. A. Bufetov, M. A. Melkumov, S. V. Firstov, A. V. Shubin, S. L. Semenov, V. V. Vel’miskin, A. E. Levchenko, E. G. Firstova, and E. M. Dianov, “Optical gain and laser generation in bismuth-doped silica fibers free of other dopants,” Opt. Lett. 36(2), 166–168 (2011). [CrossRef] [PubMed]
24. I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G. Bouwmans, and M. Douay, “Efficient all-fiber bismuth-doped laser,” Appl. Phys. Lett. 90(3), 031103 (2007). [CrossRef]
25. I. Razdobreev and L. Bigot, “On the multiplicity of bismuth active centres in germano-aluminosilicate preform,” Opt. Mater. 33(6), 973–977 (2011). [CrossRef]
26. S. V. Firstov, V. F. Khopin, I. A. Bufetov, E. G. Firstova, A. N. Guryanov, and E. M. Dianov, “Combined excitation-emission spectroscopy of bismuth active centers in optical fibers,” Opt. Express 19(20), 19551–19561 (2011). http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-19-20-19551 [CrossRef] [PubMed]
27. E. Dianov, A. Shubin, M. Melkumov, O. Medvedkov, and I. Bufetov, “High-power cw bismuth-fiber lasers,” J. Opt. Soc. Am. B 24(8), 1749–1755 (2007). [CrossRef]
28. S. Firstov, A. Shubin, V. Khopin, M. Mel’kumov, I. Bufetov, O. Medvedkov, A. Gur’yanov, and E. Dianov, “Bismuth-doped germanosilicate fiber laser with 20-W output power at 1460nm,” Quantum Electron. 41(7), 581–583 (2011). [CrossRef]
29. S. Firstov, A. Shubin, V. Khopin, I. Bufetov, A. Gur’yanov, and E. Dianov, “The 20W CW fibre laser at 1460nm based on Si-associated bismuth active centers in germanosilicate fibres,” in: Proc. of 2011 Conference on Lasers and Electro-Optics (CLEO/Europe, Munich, Germany, 2011), paper PDA7.TUE.
30. A. Luo, Z. Luo, W. Xu, V. Dvoyrin, V. Mashinsky, and E. Dianov, “Tunable and switchable dual-wavelength passively mode-locked Bi-doped all-fiber ring laser based on nonlinear polarization rotation,” Laser Phys. Lett. 8(8), 601–605 (2011). [CrossRef]
31. M. Peng, C. Zollfrank, and L. Wondraczek, “Origin of broad NIR photoluminescence in bismuthate glass and Bi-doped glasses at room temperature,” J. Phys. Condens. Matter 21(28), 285106 (2009). [CrossRef] [PubMed]
32. J. Ren, D. Chen, G. Yang, Y. Xu, H. Zeng, and G. Chen, “Near infrared broadband emission from bismuth-dysprosium codoped chalcohalide glasses,” Chin. Phys. Lett. 24(7), 1958–1960 (2007). [CrossRef]
33. G. Yang, D. Chen, W. Wang, Y. Xu, H. Zeng, Y. Yang, and G. Chen, “Effects of thermal treatment on broadband near-infrared emission from Bi-doped chalcohalide glasses,” J. Eur. Ceram. Soc. 28(16), 3189–3191 (2008). [CrossRef]
34. M. Peng, D. Chen, J. Qiu, X. Jiang, and C. Zhu, “Bismuth-doped zinc aluminosilicate glasses and glass-ceramics with ultra-broadband infrared luminescence,” Opt. Mater. 29(5), 556–561 (2007). [CrossRef]
35. M. Peng and L. Wondraczek, “Bismuth-doped oxide glasses as potential solar spectral converters and concentrators,” J. Mater. Chem. 19(5), 627–630 (2009). [CrossRef]
36. S. Zhou, W. Lei, N. Jiang, J. Hao, E. Wu, H. Zeng, and J. Qiu, “Space-selective control of luminescence inside the Bi-doped mesoporous silica glass by a femtosecond laser,” J. Mater. Chem. 19(26), 4603–4608 (2009). [CrossRef]
37. M. Peng, Q. Zhao, J. Qiu, and L. Wondraczek, “Generation of emission centers for broadband NIR luminescence in bismuthate glass by femtosecond laser irradiation,” J. Am. Ceram. Soc. 92(2), 542–544 (2009). [CrossRef]
38. G. Blasse and A. van der Steen, “Luminescence characteristics of Bi3+-activated oxides,” Solid State Commun. 31(12), 993–994 (1979). [CrossRef]
39. G. Boulon, B. Moine, and J.-C. Bourcet, “Spectroscopic properties of 3P1 and 3P0 excited states of Bi3+ ions in germanate glass,” Phys. Rev. B 22(3), 1163–1169 (1980). [CrossRef]
40. G. Boulon, B. Moine, J. C. Bourcet, R. Reisefeld, and Y. Kalisky, “Time resolved spectroscopy about 3P1 and 3P0 levels in Bi3+ doped germanate glasses,” J. Lumin. 18–19, 924–928 (1979). [CrossRef]
41. R. Reisfeld and L. Boehm, “Optical properties of bismuth in germanate, borax and phosphate glasses,” J. Non-Cryst. Solids 16(1), 83–92 (1974). [CrossRef]
42. R. Reisfeld and Y. Kalisky, “Energy transfer between Bi3+ and Nd3+ in germanate glass,” Chem. Phys. Lett. 50(2), 199–201 (1977). [CrossRef]
43. S. Parke and R. Webb, “The optical properties of thallium, lead and bismuth in oxide glasses,” J. Phys. Chem. Solids 34(1), 85–95 (1973). [CrossRef]
44. M. Peng, B. Wu, N. Da, C. Wang, D. Chen, C. Zhu, and J. Qiu, “Bismuth-activated luminescent materials for broadband optical amplifier in WDM system,” J. Non-Cryst. Solids 354(12-13), 1221–1225 (2008). [CrossRef]
45. M. Peng, C. Wang, D. Chen, J. Qiu, X. Jiang, and C. Zhu, “Investigations on bismuth and aluminum co-doped germanium oxide glasses for ultra-broadband optical amplification,” J. Non-Cryst. Solids 351(30-32), 2388–2393 (2005). [CrossRef]